Charged particle emitter apparatus



May 26, 1959 R. c. SCHMIDT ETAL CHARGED PARTICLE EMITTER APPARATUS FiledAug. 22, 1956 F i g. 3

INVENTORS Rebel-1 C, h/n1 41' y James C, Hlmer' Attorney 2,888,591CHAReED PARTICLEEMiTTER arm-Rams Robert C. Schmidt, Pale-Alto, and James(I. Filmer,

Redwood City,yCalif., assignorsto Varian Associates, SanCarlos, Calif.,a corporation o'fCalifornia Application August 22, 1 956; Serial No;605,667 9Claims. -(Cl. 3133 5) The present invention relates in generalto charged particle emitters and more specifically to a novel improvedelectron emitter o'r cath'ode useful, fo'rexa'm'ple, in obtaininghighpervearice electron beamsbtilized in velocity modulation type vacuumtubes and the "like;

Thepresent invention relates to improvements in bombarded cathodes whichare especially suitable for con tinuous, high current density emissionwherein, in addition, theemitter is subjected to high acceleratingvoltages. An'fexemplary application of such a bombarded cathode is theemitter utilized in high 'power 'klystro'n amplifiers presently utilizedfor the highpower amplification of television signals. A klystronamplifier-embodying such a cathode is taught in ac'ope'nding US. patentapplication entitled High Frequency Tube, S.N. 37 0,'568,"invented byWayneG. Abraham et al., filed-July 7, l953,now' U.S. 2,879,440.

In emitter applications where the emitter must provide continuous, highcurrent density emission into a high accelerating electric field,standard high" currentdensity emitters such'as, for example,'oxideemitters are'foundfto be unsatisfactory. These standard cathodesare'unsatisfactory'because positive ions that 'arecreated by thecollisions between the accelerated electronsof the beam and residual gasmolecules in the tube cause ions to form in the center of the beam. Then"becausethe ions have a positive charge and a strong continuouseleet'ricfieldexists between the center of the beam and'the"c'at-hodethe positive'ions will 'beaccele'rated toward the cathode; During theirflight from beam to cathode the positive ions will pick up aconsiderable amount of kinetic energy. This energy will be imparted tothe cathode emitter upon collision therewith. The efiect of the positiveion bombardmentupon theoxide cathodes is to strip the oxide coating fromthe cathode surface and thereby render the cathode inoperative; V

A pure metallic emitter is much more immune than oxide cathodes to theeffects of ion bombardment. Exaniplesof such pure metallic emitters aretantalum and tungsten. The operating temperature of a pure metallicemitter is much higher than the operating temperature'of oxide coatedcathodes. For example, oxide coated cathodes can operate at temperaturesof approximately 850 C. whereas a pure metallic emitter such as, forexample, tantalum must be operated at approximately 2000" C.

The radiation' method of heating the cathode emitter, which is generallyquite satisfactory for heating oxide coated cathodes, is generally notsatisfactory for heating pure metallic emitters. Therea'son for this isthat the hot'body from which the heat must flow has to be considerablyhotter than the cathode which it is heating or else heat will notradiate from the heater to the emitter. When the emitter operates atarelatively low temperature such as, for example, 850 C. it is easy toobtain a filament or heaterwhich 'mayoperate at a substantially highertemperaturesuch as, "for example, 1400" C. However, when the emitter ismade of a pure metallic substance 2 such as, for examplqtantalumhavingan operating temperature ot2000 C. it is difiicultif not-impossibleto-find a heater capable of withstanding the temperaturedifferential-necessary -to obtain good radiation heating.

Direct heating methods for heating the cathodes may sometimesbeemployed. The direct heating method consists of running an electricalcurrent through the cathode emitter itself thereby heating the emitterdue to the PR losses of the emitter material. Utilizing this techniqueit is easy to heat the emitter to the required operating temperature of2000 C. or higher. It will be noted that an electrical potentialgradient is obtained along the length of the'cathode emitter when it isheated utilizing the direct method. This small potential gradient arisesfrom the IR drop per unit length of the cathode emitter.

In many applications a gradient cathode is generally considered to beunsatisfactory. For example, in velocity modul'ation type-tubes such as,for example, klystrons and traveling wave tubes a unipotentia'l cathodeis normally specified to minimize the electron gun design complexity andto minimize perturbations in the electron beam. Therefore, in suchapplications the-direct heating method for heating the cathode emitteris not utilized.

A bombarding heatiug'method is the one found most satisfactory forheating pure metallic emitters. The bombarding heating method utilizes afirst electron emitter which emits the electrons into a relativelystrong accelerating field. The electrons thus emitted are acceleratedinto a beam and focusedupon the back surface of a second cathodeemitter. The heat generated by the collision of 'the firs't electronbeam with the back side of the 'cathode emitter furnishes the necessaryheat to heat the second cathode emitter.

Utilizing the bombarding technique emitter temperatures' of 2000 C. areeasily obtained. In general, this method'of heating the cathode has beenfound satisfactory providing that the electrons bombarding the cathodeemitter are properly focused onto the back side of the emitter to yielda uniform temperature and therefore a uniformemission from the frontside.

Although the positive ion bombardment difficulties were minimized by theuse of a pure metallic emitter it was found that-holes were stillappearing in the center of tantalumemitters. The cause of these holes inthe oath ode emitter is most probably three fold. Undoubtedly, one causeis the bombardment of the cathode emitter by highenergy positive ions. Asecond cause is believed to be the effect of imperfect focusing of thebombarding electron beams, which results in the concentration of heat atthe center of the button. Thirdly, the button temperature is made evencooler at the edges due to the mechanics' of, physically supporting it,and the consequent conductive cooling to the support members. Thus, thecenter of the cathode emitter operates at a higher temperature andtherefore evaporates at a faster rate than the rest of the cathodeemitter.

When a hole appears in the cathode emitter the bombarding electrons areallowed to pass therethrough and enter the main :pencil-like beam of thevelocity modulation tube. When'this takes place and A.C. hum due to theA.C. filament current appears on the main electron beam and manifestsitself as hum on the amplified RF.- signal. Inaddition, as the tubecontinues to operate the hole in'the emitter will continue to grow insize thereby decreasing the total emission. Decreasing the emissiondecreases the power output of the tubethereby shortening its usefullife.

In thepresentinvention portions, of the cathode. emitter which arelikelyto burn through ormelt are replaced by a .material having a 'morerefractory characteristic. and lower .vapor pressure therebypreventingthepositive ions and/or the poorly focused bombardingelectrons from creating a hole therein. In this manner the operatinglife of the cathode emitter is greatly extended and consequently thetube life greatly enhanced.

The principal object of the present invention is to provide a novelimproved charged particle emitter capable of providing continuous, highcurrent density emission into relatively high accelerating fields.

One feature of the present invention is the provision of a novelrefractory insert for the cathode emitter Whereby the cathode is capableof withstanding high energy bombardment for extended periods of time.

Another feature of the present invention is the provision of a taperupon the side walls of the refractory insert whereby the insert may bepressed into a mating bore in the cathode emitter thereby providing amost intimate contact between the insert and the surrounding metal suchthat both mechanical bonding and thermal effects (heat flow) aremaximized.

Other features and advantages of the present invention will be moreapparent after a perusal of the following specification taken inconnection with the accompanying drawings wherein,

Fig. l is a cross sectional view of a portion of a cathode assemblyembodying features of the present invention,

Fig. 2 is a cross sectional view of a portion of the structure of Fig. 1taken along line 2-2 in the direction of the arrows, and

Fig. 3 is an enlarged cross sectional view of a portion of the structureof Fig. 1.

Referring now to Fig. 1 there is shown a cross sectional view of aportion of the anode and cathode assembly of a high power klystronamplifier substantially identical to the klystron amplifier of theaforementioned copending patent application except for the incorporatednovel features of the present invention.

An annular concave emitter 1 as of, for example, tantalum forms theemitter portions of a Pierce type electron gun assembly. A circulardisk-like cathode center insert 1' is providde in the center of theannular cathode emitter 1. The cathode insert 1 is made of materialhaving a more refractory composition or lower vapor pressure than theannular emitter 1 such as, for example, tungsten or thoriated tungsten.The peripheral edge of the cathode insert 1 is tapered (see Fig. 3) andpressed into the center of the annular cathode emitter 1. Although 4 hasbeen shown as the degree of taper other degrees of taper should Workequally well. The press fit allows an excellent conducting junctionbetween the insert 1' and the emitter l. A good conducting junctionfacilitates the flow of heat energy through the junction and therebyaids in obtaining a uniform temperature distribution over the cathodeemitter 1.

A plurality of resilient fingers or struts 2 are fixedly secured at oneend thereof to the outside peripheral edge of the emitter 1 as by spotwelding. The resilient fingers 2 are made of a material having a highmelting point such as, for example, tantalum. The resilient fingers 2are carried tangentially at the other ends thereof by the inside wall ofa hollow cylindrical support 3 of a material having a high melting pointsuch as, for example, tantalum. The cathode support 3 also serves as aheat shield to help retain the heat energy within close proximity to thecathode emitter 1. The cathode support 3 is operated at cathodepotential.

A double spiral wound filament 4 is positioned in spaced apart relationfrom the convex face of the cathode emitter l. The filament 4 serves asthe electron emitter for supplying the electrons for bombarding the backor convex face of the cathode emitter 1 and insert 1'. The double spiralconfiguration is provided to facilitate even heating of the back surfaceof the cathode emitter 1 and insert 1'. In addition, the double woundspiral filament 4 is slightly concave shaped to further aid evenbombardment of the back surface of the cathode emitter 1 and insert l.

The filament 4 forms a direct electron emitter having its heatingcurrent supplied via two supporting filament support posts 5. Thefilament heating current is normally 60 cycle A.C. current derived fromthe 60 cycle line. The two end portions of the double spiral woundfilament 4 are bent substantially at right angles to the plane of thefilament thereby forming two filament legs. The filament legs arefixedly secured to the filament support posts 5 via two filament clips 6and two wire wraps 7.

The filament clips 6 and wire wraps 7 are made of a material having ahigh melting temperature such as, for example, molybdenum. Although thedouble spiral wound filament 4 is not a unipotential emitter thepotential gradient effects are minimized by the double spiral windingconfiguration. The operation potential of the filament 4 isapproximately 2500 volts more negative than the potential applied to thecathode emitter 1. In this manner the necessary accelerating voltagesare applied between the filamentary emitter 4 and the unipotentialcathode 1.

A filament center support post 8 is connected to the center of thedouble spiral wound filament 4 substantially at the midpoint thereof toprevent sagging of the filament 4 in use. The filament center supportpost 8 is electrically insulated from the double spiral filament 4 viaan electrical insulator (not shown). The center support post 8 is madeof a material having a high melting point such as, for example,tungsten.

A hollow cylindrical filament focus electrode 9 is positioned concentricto the double spiral filament 4- and erves a dual function of retainingthe heat energy within the filament vicinity and of focusing theelectrons emitted from the filament 4 against the back side of thecathode emitter 1 and insert ll. The filament focus electrode 9 isoperated at the same potential as the filament emitter 4.

An apertured, flat anode 1]. is positioned in spaced apart relation fromthe concave surface of the cathode emitter 1. The anode ll is providedwith a flared aperture 12 in alignment with the cathode emitter 1 tofacilitate focusing of the emitted electrons into a beam. The electronbeam passes through the flared aperture without appreciable beaminterception. The anode operates at a high positive potential withrespect to the potential of the cathode emitter, for example, 17,500volts more positive.

A flared hollow cylindrical cathode focus electrode 13 is positionedconcentric to the cathode emitter 1 and in overhanging spatial relationthereto to aid in focusing the emitted electrons through the aperture 12in anode 11. A hollow cylindrical cathode focus support 14 serves tocarry the cathode focus electrode 13. The cathode focus electrode 13 andsupport 14 operate at the same potential as the cathode emitter 1 andboth are made of a material having a high melting temperature such as,for example, tantalum.

A tubular member 15 forming a portion of the outer cathode envelope ispositioned concentric of the inner cathode assembly and is carried bythe anode 11. The tubular member 15 is made of a non-magnetic materialsuch as, for example, copper to prevent perturbation of the magneticfocusing field in this region. A second thin walled tubular member 16 asof Kovar is carried upon one end of the tubular member 15 and is coupledto a hollow cylindrical dielectric insulator 17 as of glass. The thintubular Kovar member 16 is made yieldable to allow for thermally causedexpansion and contraction of the dielectric insulator 17 in use. Theinsulator 17 serves to insulate the anode 11 from the much lower voltageapplied to the cathode assembly.

In operation electrons are emitted from the filament 4 and focused upona back side of the cathode emitter 1 including the cathode insert 1'.Since the potential difference between filament 4 and cathode emitter 1and 1' is approximately 2500 volts the electrons bombarding the backside of the emitter transform their kinetic energy into heat energy. Inthis manner the operating temperature of the cathode emitter 1 andinsert 1' are brought up to approximately 2000 C. which allows thecathode 1 to become electron emissive.

The electrons emitted by the concave cathode emitter 1 are drawn awayfrom the cathode 1 and focused into a beam due to the combined action ofthe cathode focus electrode 13 and anode 11. The beam passes through theflared aperture 12 in the anode 11 and passes into the energyinteraction area of the vacuum tube.

Positive ions created due to the collision of electrons Within the beamand residual gas molecules within the tube collect in the center of theelectron beam. Upon collecting the positive ions are accelerated towardthe cathode emitter 1 and insert 1' and focused thereupon due to wellknown physical phenomena. The bombarding positive ions having obtainedconsiderable kinetic energy collide upon the cathode 1 and insert 1' andcause additional heating thereof. Due to the edge cooling of the cathode1 by the supporting resilient fingers 2 and cathode support 3 and due toimperfect focusing of the bombarding electrons the center of the cathodebecomes somewhat hotter than the edge. However, due to the morerefractory composition of the center insert 1' and lower vapor pressureof the insert 1 and due to the good heat conducting junction between theinsert 1' and the annular emitter 1 adverse effects caused byoverheating of the cathode are eliminated. It is believed that when theannular cathode emitter 1 is made of tantalum and the center insert 1'is made of tungsten emission takes place from the center insert 1' aswell as from the annular emitter 1 thereby minimizing perturbations ofthe electron beam which might arise from a hollow beam.

Since many changes could be made in the above construction and manyapparently widely difierent embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An electron emitter apparatus for applications where portions of theemitter are subjected to higher temperatures than other portionsincluding, a first emitter member for providing high current densityemission at normal operating temperatures in excess of 1500" C., asecond member made of a material having a more refractory compositionthan said first member and disposed within said first emitter member inthe areas of higher average temperature thereof whereby the deleteriouseffects of the higher temperature may be counteracted and the operatinglife of the electron emitter greatly extended.

2. In an apparatus as claimed in claim 1 wherein said first emittermember comprises a portion of a concave emitter.

3. In an apparatus as claimed in claim 1 wherein said first emittermember comprises a portion of a substantially unipotential emitter.

4. In an apparatus as claimed in claim 1 wherein said first and secondmembers comprise portions of a substantially unipotential emitter.

5. In an apparatus as claimed in claim 4 wherein said second refractorymember is centrally disposed within said emitter member and incontiguous relationship thereto.

6. In an apparatus as claimed in claim 4 wherein said first emittermember comprises a substantially annular segment of a sphere, and saidsecond refractory member comprises a substantially circular disc mountedwithin and closing over the central aperture in said first annularemitter member.

7. In an apparatus as claimed in claim 6 wherein said second refractorymember of substantially circular configuration is provided with atapered peripheral edge for press fitting within the central aperture ofsaid first annular emitter member to thereby facilitate mechanicalbonding and heat fiow through the bond to minimize deleterious effectsproduced by local concentrations of heat energy.

8. In an electron discharge apparatus utilizing a high current densityelectron beam for interaction with high frequency electromagnetic fieldsand having a cathode assembly including a first high current densityemitter for supplying the electrons making up the beam and said firstemitter being disposed in axial alignment with the beam of electrons, afilamentary emitter member disposed in spaced apart relation from saidfirst high current density emitter on the side thereof remote from theelectron beam and serving as a source of electrons for bombarding saidfirst emitter member, the bombarding electrons serving to heat saidfirst emitter member to an operating temperature in excess of 1500 C., atungsten member disposed in axial alignment With the electron beam andadjacent said first high current density emitter, and said tungstenmember having a more refractory composition and lower vapor pressurethan that portion of said first emitter immediately adjacent to saidtungsten member to thereby prevent damage to said first emitter causedby local concentrations of heat in the vicinity of said, first emitterand said tungsten member.

9. In an apparatus as claimed in claim 8 wherein said first emittermember comprises an annular tantalum segment of a sphere, said tungstenmember comprises a circular disc mounted within a central aperture insaid first emitter member, and said filamentary emitter comprises adouble spiral wound filament having a concave shape to facilitate evenheating of said first emitter member.

References Cited in the file of this patent UNITED STATES PATENTS2,398,829 Haeff Apr. 23, 1946 2,410,822 Kenyon Nov. 12, 1946 2,569,872Skehan et al. Oct. 2, 1951 ,573,287 Szegho Oct. 30, 1951 2,751,514 AtleeJune 19, 1956

